The present invention relates to a platform shake compensation method in a synthetic aperture processing system of radar or sonar.
Heretofore, in general, it is known that the shake of a platform mounting a synthetic aperture processing system adversary affects a synthetic aperture result. In the case of synthetic aperture radar using an electric wave, a moving velocity of the platform is far slower than a rate of propagation of an electric wave, so that the effect of the shake is small. Hence, if angle compensation and geometry compensation are performed in almost all the cases, a sufficiently detailed image can be acquired, and moreover, even in case the system is moved by an aircraft under bad weather, if the shake compensation is performed by using positional information by GPS (Global Position System) and INS (Inertial Navigation System), the situation can be coped with. However, in the case of synthetic aperture sonar using a sound wave, a moving velocity of the platform is relatively close to a rate of propagation of a sound wave, and moreover, it is often the case that an ocean wave gives to the platform a shake of a large vibration amplitude, and therefore, under the circumstances, it takes a lot more than the compensation used by the synthetic aperture radar to be sufficient to cope with the situation.
When the shake compensation method using the positional information is applied particularly to the synthetic aperture sonar, what becomes an issue is accuracy of the positional information. Accuracy of a position found by the synthetic aperture sonar is generally several centimeters to several tens of centimeters. However, accuracy of the position of the GPS is several meters or more, and accuracy of the position of the INS is unable to acquire accurate positional information independently due to its configuration. When shake compensation is performed by using inaccurate positional information, its position error also adversely affects the synthetic aperture result, and in case the position error is larger than the actual shake, the result after the compensation becomes much worse. At present, there is no measurement apparatus which can satisfy the positional accuracy required by the synthetic aperture sonar, and therefore, compensation processing is not effectively functioned. Hence, the synthetic aperture sonar under test working is limited to such extent of achieving a limited success in a lake or gulf where oceanographic phenomenon is mild and the shake of the platform is extremely little. Further, in the case of the synthetic aperture radar also, though the effect of the shake is little, it makes no difference as it still remains as a problem.